Mixing machines for use in mixing fireproofing material, concrete, plaster, and the like, are well known and often include a generally cylindrical-shaped mixing drum mounted for pivotal rotation about a generally horizontal axis between normal and dumping positions. A shaft with attached mixer blades extends along the horizontal pivot axis of the mixer drum, which shaft is rotatably driven for rotation of the mixer blades for mixing of the drum contents. Opening means are formed at the upper wall of the drum through which the material to be mixed is dumped into the mixer drum, and from which the mix is discharged. After mixing of the material with liquid, the mixing drum is pivoted to dumping position for dumping the mix through the discharge opening. An open net or grill structure sometimes is used to cover the opening means in the drum to prevent the operator from engaging the mixer blades with his hands while still allowing material to be dumped into the mixing drum and mixed material to be discharged therefrom. The disclosure contained in U.S. Pat. No. 2,987,302 by P. A. Terry et al exemplifies one prior art mixing machine of this general type.
A major problem with prior art mixing machines, including the Terry et al machine, is that clouds of dust particles issue from the machine as the contents of the sacks of material to be mixed are dumped into the mixer drum, and during the mixing process. The surrounding atmosphere is contaminated with the material, imperiling the health of persons breathing the same, including that of the mixer operator. Often the mixer operator is required to wear an air-filtering mask to limit the amount of material inhaled, and strict standards promulgated under the Occupational Safety and Health Act drastically limit exposure of workers to such hazards.